Plasma etching and related processes such as reactive ion etching (RIE) are used in semiconductor device processing to transfer specific patterns into underlying films. Typically, such patterns are defined in an overlying layer of photoresist and, through the use of an etching process, transferred into an underlying film. Such an underlying film can comprise, for example, one of the insulating or conducting films typically utilized over semiconductor materials in the formation of semiconductor devices.
As part of the process of transferring a pattern into an underlying film, it is necessary to stop the etching process at a specific depth. This depth is typically, but not always, equal to the total depth of the underlying film. The point at which the specific depth has been reached is called in the art the endpoint of the etching process. The accuracy with which this depth is obtained and the completeness with which the film is removed have a direct impact on the performance of the subsequently completed device. If, for example, an undesired residue of film is left in a transistor, it can result in an undesirably high resistance value for a particular transistor region. To further complicate the process of transferring a pattern, device topology as well as previous processing steps can cause the thickness of the film being etched to vary substantially across the object being etched.
Several methods are currently known for detecting an endpoint during etching.
The process of Optical Emission Spectroscopy (OES) operates by detecting the wavelength of light emitted by a specific chemical species, usually a species of a by-product of the etching process. Using a monochromator, this specific wavelength is isolated by filtering out all other wavelengths. The energy in this isolated, filtered light is converted into a specified unit of energy, for example into volts through the use of a detector. The detector is used to monitor the magnitude of the energy through, for example, a strip-chart reader or a computer monitor. A predetermined change in the light energy is used to detect the endpoint.
OES is advantageous in that no endpoint site is required. It also, however, entails several disadvantages, including: (1) the understanding of complex plasma chemistry whereby to select a chemical species for monitoring; (2) a high dependence on pattern density (etched area to unetched area), whereby a low pattern density (e.g. 20% or less) produces very little of the chemical species being monitored, making detection of the endpoint difficult; (3) a limited selection of gases for etching so as to avoid generating a chemical species similar to the endpoint species; and (4) confusing chemical species generated from the inside of the etching chamber.
U.S. Pat. No. 4,611,919 to Brooks, Jr. et al. shows a process for detecting an endpoint by monitoring a change in a voltage proportional to the total intensity of energy reflected from a wafer during an etching process. This method is disadvantageous in requiring a very high etch rate ratio of the overlying film to the underlying film (i.e. high selectivity). This method is also sensitive to pattern density in the manner described above with respect to OES.
U.S. Pat. No. 4,198,261 to Busta et al. shows a method utilizing interferometry for endpoint detection. This method monitors the interference fringes produced as a result of multiple reflections due to the different refractive indices of two films. Each interference fringe corresponds to a known thickness of the film that has been etched. This method has the advantage of being insensitive to pattern density or to the etching gasses used.
The method of Busta et al., however, as typically used in the art, has the disadvantage of requiring an endpoint detection area, termed a fiducial site (or area, region, or mark), to detect an endpoint. Such a fiducial site comprises at least one dedicated die area for detecting an endpoint. Such a fiducial site requires an overlying film which must be removed prior to the etching of an underlying film. Where, for example, the overlying film is photoresist, it is removed by exposure and subsequent development. The use of a fiducial site suffers from the well-known disadvantages of requiring extra masking steps, alignment steps, and exposure steps in addition to the device fabrication process steps.
U.S. Pat. No. 4,687,539 to Burns et al. shows a process wherein an excimer laser is used to burn/etch through a film, and the endpoint is detected by analyzing the wavelength of the flouresced, vaporized by-product. This endpoint detection is similar to the OES process described hereinabove. Burns et al. has the many known disadvantages inherent in the use of a laser for etching, including substantial material and process limitations.
Briefly: U.S. Pat. No. 4,479,848 to Otsubo et al. shows an etching process wherein a change in contrast of a dicing stripe pattern is used to determine an endpoint; U.S. Pat. No. 4,496,425 to Kuyel shows a process wherein the intensity of light reflected from a Fresnel zone plate is used to determine an endpoint; and U.S. Pat. No. 4,717,446 to Nagy et al. shows a method wherein a separate monitor wafer is used to measure the etch rate and determine an endpoint.